1. Field of the Invention
This invention relates to plethysmography and forced oscillation respiratory impedance testing of humans to determine respiratory characteristics.
2. Description of the Prior Art
Measurements of human respiratory system impedance using a forced oscillation technique can provide quantitative insight into the mechanical properties of the respiratory system. Typically, the impedance spectra are interpreted using an electromechanical model with proposed physiological parameters. The most widely used electromechanical model is the six-element model proposed by DuBois et al. in 1956 which allows a lumped separation of airways and tissues. DuBois et al. introduced two methods of non-invasively measuring the mechanical properties of the human respiratory system. In one method, small amplitude pressure oscillations are applied at the airway opening (mouth) and the resulting air pressure and air flow at the airway opening are measured. The ratio of the pressure to flow at the airway opening (P.sub.ao /V.sub.ao) is termed input impedance (Z.sub.in). In the second method, the pressure oscillations are applied at the chest wall (P.sub.cw). The pressure applied to the chest and the air flow at the airway opening (V.sub.ao) are measured. The ratio (V.sub.ao) is termed transfer impedance (Z.sub.tr).
To analyze respiratory system impedance data, DuBois et al. proposed a three compartment, six element model based on the following assumptions: 1) the lung is a monoalveolar compartment that can be represented by a simple gas compression, 2) the model parameters are frequency independent, and 3) the airways are noncompliant structures. As shown in FIG. 1, this model comprises an airway impedance compartment (Z.sub.aw) comprising an airways resistance (R.sub.aw) in series with an airways inertance (I.sub.aw). The tissue impedance compartment (Z.sub.ti) is modeled as a tissue resistance (R.sub.ti) in series with a tissue inertance (I.sub.ti) and a tissue compliance (C.sub.ti). These two compartments are separated by a shunt gas compression compartment (Z.sub.g) which is modeled as a simple gas compression term (C.sub.g).
From this model the transfer impedance (Z.sub.tr) for the DuBois model is given by Equation 1 below: ##EQU1##
As Peslin et al. pointed out, though, in using this DuBois model to analyze Z.sub.tr, it is necessary to independently measure one of the six element parameter values. The most common practice has been to measure functional residual capacity (FRC) and then calculate C.sub.g from Equation 2 below: ##EQU2##
wherein P.sub.ATM =atmospheric pressure (1033 cm H.sub.2 O) and P.sub.H2O =partial pressure of water vapor at 100% saturation (64 cmH.sub.2 O).
In order to perform transfer impedance testing, Peslin et al. disclosed a body box which completely encloses the subject and provides a tube for connecting the air supply and flow measurement devices to the mouth of the subject inside of the box. Peslin et al. also disclosed the use of a signal generator connected to a loudspeaker to provide the pressure to the subject and a computer to analyze the data collected using a six element model. Because the box entirely encloses the patient, however, some patients were apprehensive about this type of testing.
Most studies of human transfer impedance have been limited to frequency ranges of 4-30 Hz. Others have used frequencies up to 64 Hz.